The present invention relates to semiconductor photodetectors and more particularly to silicon photodetectors used for detecting ultraviolet radiation.
In the past ultraviolet (UV) detection by silicon photodiodes has met with many difficulties. A photodetector is a device which responds to radiant energy by generating carriers which are collected as current. As example of a photodetector is a photodiode.
In the ultraviolet region of the electromagnetic spectrum, i.e., 1,000 up to 4,000 angstroms (A) wavelength, radiation absorption by silicon is very high, i.e., typically greater than 10.sup.5 /cm, leading to complete attenuation of the ultraviolet radiation within about 1,000A or less of the silicon body incident surface. If the UV generated carriers are to be detected they must diffuse to the depletion region of the photodiode where the electric field can sweep them across the junction for collection as current. Surface physics dictate that the properties of the silicon body near a surface decreases the lifetime of the carriers generated near that surface and increases the likelihood of recombination and trapping of these generated carriers. If the depletion region in the conventional silicon photodetector is extended all the way to the surface, noise and leakage current due to carrier injection are considerably enhanced. In addition, it is difficult to passivate the surfaces of silicon photodetectors if UV detection is contemplated, since the oxides of silicon which would be used as a passivation layer will themselves absorb ultraviolet radiation. Indeed, the surfaces of the silicon body will typically have to be left entirely exposed because all optical materials which might be used in hermetically passivating such a body are themselves absorbing in the ultraviolet radiation range. The same applies to antireflection coating materials so that an additional problem encountered is that there will be very high reflectivity of the silicon body in the UV radiation range and subsequent losses in detection sensitivity.
Furthermore, many conventional silicon ultraviolet detectors made by conventional diffusion techniques show degradation of performance with exposure to ultraviolet radiation. This problem is due to silicon damage which is occasioned by the high energy photons of ultraviolet radiation. Therefore, it would be most desirable in the silicon photodetector field if a silicon body could be modified for UV detection with the elimination of the problems of high reflectance, high noise, leakage current, degradation and environmental contamination.